1. Field of the Invention
The present invention relates generally to batteries for spacecraft applications and, more particularly, to a system for minimizing the impact of a failed battery cell achieved by operating on the battery circuit so as to optimize the usable battery energy storage capacity after the failure occurs.
2. Description of the Prior Art
The effect of a failed cell on the capacity of the remainder of the cells may also be minimized by using a large number of battery cells connected in parallel with fuse protection. Such a technique is presented in commonly assigned co-pending U.S. application Ser. No. 08/955,931 assigned to Space Systems/Loral, Inc., Docket No. PA-96090 entitled "Massively Parallel Spacecraft Battery Cell Module Design".
A key point to be considered is that for a lithium ion battery in a series-parallel array, a failed cell in one series string of battery cells of the array undesirably reduces the allowed charging voltage of the remaining series strings of the array and thereby reduces their capacity. This invention avoids such an undesirable result by sacrificially removing cells from all series strings of the array.
By way of example, for a 100 V lithium ion battery, the invention leads to a 4% capacity loss if one cell fails. Operating all arrays at a depressed voltage leads to a 50% loss while removing the effected string results in a more moderate 25% loss.
As background, it must be appreciated that spacecraft reliability requirements entail that no function on the satellite can be vulnerable to the failure of a single component. Typically, this requirement is satisfied by having multiple redundant components for each function. For batteries, the requirement is satisfied today by including an additional cell in each series string. All battery cells in the string are equipped with bypass electronic components (e. g. diodes or switches). In the event that a battery cell fails or short circuits, the battery operates with a one cell capacity reduction for which it was originally sized. In the event that the cell fails, open circuit current continues to be passed through the series string by way of the bypass circuitry.
While the invention is generally applicable to all styles of batteries, the situation with lithium ion batteries is somewhat unique in that they are not presently available in cells of sufficient size to meet the requirements of advanced high power satellites with single series string designs. Even if such lithium ion battery cells of appropriate size were available, reengineering of cells to match capacity requirements is not a desirable exercise. These factors make the use of smaller building block cells in a series-parallel combination attractive.
Two possible approaches for arranging building block cells are illustrated in FIG. 1. In a parallel-series array 20, as illustrated, individual cells 22 are connected first in parallel to form a cell module 24 and then in series along a lead 26 to form a high voltage battery unit. In a series-parallel array 28 as seen in FIG. 2, series strings 30, 32, and 34 of cells 36 are connected in parallel.
The parallel-series array 20 is electrically identical to commonly employed present satellite battery practice and similar bypass strategies may be employed. The series-parallel array presents a different problem owing to the narrow cell voltage range (illustrated in FIG. 3) over which charging occurs for a lithium ion cell and the high individual cell voltage of a lithium ion type battery. Referring to FIG. 3, it is seen that approximately half the capacity (from approximately 600 mAh to 1200+ mAh) is charged into the cell as the voltage increases. from 4.1 to 4.2 V. For a 25 cell battery, this corresponds to a voltage change of 2.5 V.
In a 25 cell series string, if one cell fails, its string charging voltage is reduced by 4.2 V/25=0.17 V. per cell.
Based on FIG. 3, this operation reduces the capacity of the parallel-series strings from 1250 mAh to 300 mAh, a 75% reduction. In addition, it creates an instability in the battery system as the string with the failed cell will have 4 times the coulombic capacity as compared to the parallel strings with no failed cells. Potential solutions to this problem are:
bypass the entire series string; unless the number of strings exceeds the number of cells per string, however, this is a bad trade; for example a 10 kW, 100 V, 73% DOD ("depth of discharge") satellite battery built with 60 Ah cells requires three 24 cell series strings to meet the capacity requirements; single point failure invulnerability requires an additional series string which is a 33% overbuild of the battery; PA1 individually regulate the voltage on each string; even for a regulated bus, this is an unattractive option as component redundancy requirements in the regulator will adversely increase mass in a distributed system; it also precludes an unregulated bus which is gravimetrically superior to the regulated bus.
It was with knowledge of the foregoing that the present invention has been conceived and is now reduced to practice.